Add comment to mock-compiler.
Add comment to mock-compiler.

Sample backtrace:
#0      Object.noSuchMethod (dart:core-patch:600:3)
#1      PartialTypeTree.newNode (file:///dart/lib/compiler/implementation/universe/partial_type_tree.dart:47:52)
#2      PartialTypeTree.findNode (file:///dart/lib/compiler/implementation/universe/partial_type_tree.dart:76:21)
#3      SelectorMap.[]= (file:///dart/lib/compiler/implementation/universe/selector_map.dart:25:39)
#4      ArgumentTypesRegistry.registerDynamicInvocation (file:///dart/lib/compiler/implementation/js_backend/backend.dart:520:22)
#5      JavaScriptBackend.registerDynamicInvocation (file:////dart/lib/compiler/implementation/js_backend/backend.dart:774:44)
#6      SsaCodeGenerator.visitInvokeClosure (file:///dart/lib/compiler/implementation/ssa/codegen.dart:1556:38)
#7      HInvokeClosure.accept (file:///dart/lib/compiler/implementation/ssa/nodes.dart:1323:57)

Review URL: https://codereview.chromium.org//11184042

git-svn-id: c93d8a2297af3b929165606efe145742a534bc71@13892 260f80e4-7a28-3924-810f-c04153c831b5

dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk