Better error messages

Better error messages

This records the actual point at which type inference fails and uses that to give a better error message.  So where before we would say:

severe: line 76, column 38 of package:angular2/src/forms/directives.dart: [StaticTypeError] Type check failed: [c.validator, this.validator] (List<dynamic>) is not of type List<Function>
    c.validator = validators.compose([c.validator, this.validator]);

we will now in addition say:

because c.validator cannot be typed as Function

BUG=
R=vsm@google.com

Committed: https://github.com/dart-lang/dev_compiler/commit/5ebda81983a16422298f00ffefc3b6b366e6de3e

[Leaf, 2015-04-03 19:37:38]

leafp@google.com changed reviewers:
+ vsm@google.com

[vsm, 2015-04-03 19:48:26]

lgtm!

[Leaf, 2015-04-03 19:49:08]

Committed patchset #1 (id:1) manually as
5ebda81983a16422298f00ffefc3b6b366e6de3e (presubmit successful).