This addition is related to the definition of olx.FrameState in olx.js. Without this alias and corresponding goog.provide in src/ol/vec/mat4.js we get this error when compiling an OpenLayers application:
ERR! compile node_modules/openlayers/externs/olx.js:6853: ERROR - Bad type annotation. Unknown type goog.vec.Mat4.Number
ERR! compile * coordinateToPixelMatrix: goog.vec.Mat4.Number,
Currently, the Closure Library does not have a goog.provide for goog.vec.Mat4.Number. If it did have this goog.provide then we wouldn't have the error.
This commit works around the issue by creating our own type alias (ol.vec.Mat4.Number) and adding a goog.provide for that type alias.
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