Add message to assertions.

src/ol/renderer/canvas/canvasimagelayerrenderer.js

@@ -156,7 +156,8 @@ ol.renderer.canvas.ImageLayer.prototype.prepareFrame =
 
   var image;
   var imageLayer = this.getLayer();
-  goog.asserts.assertInstanceof(imageLayer, ol.layer.Image);
+  goog.asserts.assertInstanceof(imageLayer, ol.layer.Image,
+      'layer is an instance of ol.layer.Image');
   var imageSource = imageLayer.getSource();
 
   var hints = frameState.viewHints;
@@ -172,7 +173,8 @@ ol.renderer.canvas.ImageLayer.prototype.prepareFrame =
     var projection = viewState.projection;
     var sourceProjection = imageSource.getProjection();
     if (!goog.isNull(sourceProjection)) {
-      goog.asserts.assert(ol.proj.equivalent(projection, sourceProjection));
+      goog.asserts.assert(ol.proj.equivalent(projection, sourceProjection),
+          'projection and sourceProjection are equivalent');
       projection = sourceProjection;
     }
     image = imageSource.getImage(

src/ol/renderer/canvas/canvaslayerrenderer.js

@@ -50,7 +50,8 @@ ol.renderer.canvas.Layer.prototype.composeFrame =
     var extent = layerState.extent;
     var clipped = goog.isDef(extent);
     if (clipped) {
-      goog.asserts.assert(goog.isDef(extent));
+      goog.asserts.assert(goog.isDef(extent),
+          'layerState extent is defined');
       var pixelRatio = frameState.pixelRatio;
       var topLeft = ol.extent.getTopLeft(extent);
       var topRight = ol.extent.getTopRight(extent);

src/ol/renderer/canvas/canvasmaprenderer.js

@@ -83,7 +83,7 @@ ol.renderer.canvas.Map.prototype.createLayerRenderer = function(layer) {
   } else if (ol.ENABLE_VECTOR && layer instanceof ol.layer.Vector) {
     return new ol.renderer.canvas.VectorLayer(layer);
   } else {
-    goog.asserts.fail();
+    goog.asserts.fail('unexpected layer configuration');
     return null;
   }
 };
@@ -138,7 +138,8 @@ ol.renderer.canvas.Map.prototype.dispatchComposeEvent_ =
  */
 ol.renderer.canvas.Map.prototype.getCanvasLayerRenderer = function(layer) {
   var layerRenderer = this.getLayerRenderer(layer);
-  goog.asserts.assertInstanceof(layerRenderer, ol.renderer.canvas.Layer);
+  goog.asserts.assertInstanceof(layerRenderer, ol.renderer.canvas.Layer,
+      'layerRenderer is an instance of ol.renderer.canvas.Layer');
   return /** @type {ol.renderer.canvas.Layer} */ (layerRenderer);
 };
 
@@ -185,7 +186,8 @@ ol.renderer.canvas.Map.prototype.renderFrame = function(frameState) {
     layerState = layerStatesArray[i];
     layer = layerState.layer;
     layerRenderer = this.getLayerRenderer(layer);
-    goog.asserts.assertInstanceof(layerRenderer, ol.renderer.canvas.Layer);
+    goog.asserts.assertInstanceof(layerRenderer, ol.renderer.canvas.Layer,
+        'layerRenderer is an instance of ol.renderer.canvas.Layer');
     if (!ol.layer.Layer.visibleAtResolution(layerState, viewResolution) ||
         layerState.sourceState != ol.source.State.READY) {
       continue;

src/ol/renderer/canvas/canvastilelayerrenderer.js

@@ -182,7 +182,8 @@ ol.renderer.canvas.TileLayer.prototype.prepareFrame =
   var projection = viewState.projection;
 
   var tileLayer = this.getLayer();
-  goog.asserts.assertInstanceof(tileLayer, ol.layer.Tile);
+  goog.asserts.assertInstanceof(tileLayer, ol.layer.Tile,
+      'layer is an instance of ol.layer.Tile');
   var tileSource = tileLayer.getSource();
   var tileGrid = tileSource.getTileGridForProjection(projection);
   var tileGutter = tileSource.getGutter();
@@ -218,9 +219,12 @@ ol.renderer.canvas.TileLayer.prototype.prepareFrame =
 
   var canvas, context;
   if (goog.isNull(this.canvas_)) {
-    goog.asserts.assert(goog.isNull(this.canvasSize_));
-    goog.asserts.assert(goog.isNull(this.context_));
-    goog.asserts.assert(goog.isNull(this.renderedCanvasTileRange_));
+    goog.asserts.assert(goog.isNull(this.canvasSize_),
+        'canvasSize is null (because canvas is null)');
+    goog.asserts.assert(goog.isNull(this.context_),
+        'context is null (because canvas is null)');
+    goog.asserts.assert(goog.isNull(this.renderedCanvasTileRange_),
+        'renderedCanvasTileRange is null (because canvas is null)');
     context = ol.dom.createCanvasContext2D(canvasWidth, canvasHeight);
     this.canvas_ = context.canvas;
     this.canvasSize_ = [canvasWidth, canvasHeight];
@@ -228,8 +232,10 @@ ol.renderer.canvas.TileLayer.prototype.prepareFrame =
     this.canvasTooBig_ =
         !ol.renderer.canvas.Layer.testCanvasSize(this.canvasSize_);
   } else {
-    goog.asserts.assert(!goog.isNull(this.canvasSize_));
-    goog.asserts.assert(!goog.isNull(this.context_));
+    goog.asserts.assert(!goog.isNull(this.canvasSize_),
+        'non-null canvasSize (because canvas is not null)');
+    goog.asserts.assert(!goog.isNull(this.context_),
+        'non-null context (because canvas is not null)');
     canvas = this.canvas_;
     context = this.context_;
     if (this.canvasSize_[0] < canvasWidth ||
@@ -277,7 +283,8 @@ ol.renderer.canvas.TileLayer.prototype.prepareFrame =
     canvasTileRangeWidth = canvasTileRange.getWidth();
   }
 
-  goog.asserts.assert(canvasTileRange.containsTileRange(tileRange));
+  goog.asserts.assert(canvasTileRange.containsTileRange(tileRange),
+      'tileRange is contained in canvasTileRange');
 
   /**
    * @type {Object.<number, Object.<string, ol.Tile>>}

src/ol/renderer/canvas/canvasvectorlayerrenderer.js

@@ -132,7 +132,7 @@ ol.renderer.canvas.VectorLayer.prototype.forEachFeatureAtCoordinate =
          * @return {?} Callback result.
          */
         function(feature) {
-          goog.asserts.assert(goog.isDef(feature));
+          goog.asserts.assert(goog.isDef(feature), 'received a feature');
           var key = goog.getUid(feature).toString();
           if (!(key in features)) {
             features[key] = true;
@@ -161,7 +161,8 @@ ol.renderer.canvas.VectorLayer.prototype.prepareFrame =
     function(frameState, layerState) {
 
   var vectorLayer = /** @type {ol.layer.Vector} */ (this.getLayer());
-  goog.asserts.assertInstanceof(vectorLayer, ol.layer.Vector);
+  goog.asserts.assertInstanceof(vectorLayer, ol.layer.Vector,
+      'layer is an instance of ol.layer.Vector');
   var vectorSource = vectorLayer.getSource();
 
   this.updateAttributions(