Is the mat4(mat3) constructor defined as part of GLES2.0's shader language? If it isn't then an implementation (such as ANGLE) should be explicitly disallowing it on all platforms. If it is defined as part of the spec then an implementation must ensure that it will work regardless of the backend. If it's not part of the GLSL ES spec, then we would be extending the language, and so need to define that extension, but i'm fairly sure we don't want to do anything like that.
Also try to recall that ANGLE is simply one (popular) validation tool used by WebGL implementations, so it can't just be arbitrarily extended beyond what the specifications allows.