Make implicit casts hints

Make implicit casts hints

This appears to cut out about 1/3 of the strong mode errors reported
in angular2.  The actual cases removed that I looked at seem fine - e.g., we may inject an implicit cast, but it'll work fine at runtime.

Fixes #302

R=leafp@google.com

Committed: https://github.com/dart-lang/dev_compiler/commit/a7533099d6fd5b0a216aa4c001e4199dcfe92a6b

[vsm, 2015-08-26 21:21:37]

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

[Leaf, 2015-08-26 21:25:22]

lgtm

[yjbanov, 2015-08-26 21:26:23]

On 2015/08/26 21:21:41, vsm wrote:

What would be DDC's output when it encounters an implicit downcast? Would it
make call sites dynamic or would it inject a runtime cast check?

[vsm, 2015-08-26 21:36:07]

On 2015/08/26 21:26:23, yjbanov wrote:
> On 2015/08/26 21:21:41, vsm wrote:
> 
> What would be DDC's output when it encounters an implicit downcast? Would it
> make call sites dynamic or would it inject a runtime cast check?

It would inject a runtime check - a following call site would not be dynamic.

[vsm, 2015-08-26 22:02:08]

Committed patchset #1 (id:1) manually as
a7533099d6fd5b0a216aa4c001e4199dcfe92a6b (presubmit successful).