getExtension() of WebGLRenderingContext is broken

[DartBot]

This issue was originally filed by @pjako

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What steps will reproduce the problem?

1. Run my test code.... (see below)

What is the expected output?
_getExtension should return an instance WebGLCompressedTextureS3TC or null
What do you see instead?
Dartium version of the windows bluescreen ;)

What version of the product are you using? On what operating system?
Windows 7 64
SDK 8370

Please provide any additional information below.

The testcode:

import('dart:html');

final List<String> vendorPrefixes = const ["", "WEBKIT", "MOZ_"];
void main() {
    var can = new CanvasElement();
    
    var gl = can.getContext("experimental-webgl");
    if(gl == null) can.getContext("webgl");
    _getExtension(gl, "WEBGL_compressed_texture_s3tc") != null;
}
Object _getExtension(WebGLRenderingContext gl, String name) {
    var ext;
    for(String vendor in _vendorPrefixes) {
      ext = gl.getExtension("$vendor$name");
      return null;
      if (ext != null) {
        return ext;
    }
  }
  return null;
}

[dgrove]

Set owner to antonm@google.com.
Added Area-Dartium, Triaged labels.

[DartBot]

This comment was originally written by antonm@google.com

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Added this to the M1 milestone.

[DartBot]

This comment was originally written by antonm@google.com

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Added Fixed label.