Enable is and as checks on non-ground types

Enable is and as checks on non-ground types

Failures generate a StrongModeError.  Fixes #236.  Partially addresses #238.

Some issues to do:
- We're too eager to throw a StrongModeError.  E.g., <int>[] is List<String> will throw.
- We don't differentiate between implicit and explicit casts.

R=leafp@google.com

Committed: https://github.com/dart-lang/dev_compiler/commit/1aa9fe9760a501874360040012f9901fd66b11f6

[vsm, 2015-08-18 22:56:20]

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

[Leaf, 2015-08-18 23:31:12]

lgtm with comment.

https://codereview.chromium.org/1298893003/diff/20001/lib/runtime/_operations.js
File lib/runtime/_operations.js (right):

https://codereview.chromium.org/1298893003/diff/20001/lib/runtime/_operations...
lib/runtime/_operations.js:212: dart_utils.throwStrongModeError('Strong mode
cast failure from ' +
Calling this a "cast failure" is a bit misleading, I think.  Maybe call it an
"is check failure", and say something like "types.typeName(actual) is not an
instance of types.typeName(type)"?

[vsm, 2015-08-19 13:15:14]

Committed patchset #2 (id:20001) manually as
1aa9fe9760a501874360040012f9901fd66b11f6 (presubmit successful).