fixes strong mode assignability

fixes strong mode assignability
this fixes https://github.com/dart-lang/dev_compiler/issues/356

The issue here is that StrongTypeSystemImpl has toType and fromType backwards from how they are defined on the base class.
The way it works is to read it as "is the first type assignable to the second type"?

(I can see how this may have happened: the base class uses "leftType" and "rightType", which refers to the parameter order, not the order they would appear in an assignment expression.)

R=leafp@google.com

Committed: https://github.com/dart-lang/sdk/commit/a8570d9e7cfe2427a2bd99348b9ae8199a5903ff

[Jennifer Messerly, 2015-10-01 22:21:42]

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

[Jennifer Messerly, 2015-10-01 22:24:15]

Here's an example of how ErrorVerifier._checkForInvalidAssignment(lhs, rhs) uses
this method, to illustrate the problem:

if (!_typeSystem.isAssignableTo(staticRightType, leftType)) {
      _errorReporter.reportTypeErrorForNode(
          StaticTypeWarningCode.INVALID_ASSIGNMENT,
          rhs,
          [staticRightType, leftType]);
...
}

note how we're checking if the assignment right side (the "from type") is
assignable to the assignment left side (the "to type")

https://codereview.chromium.org/1380963004/diff/1/pkg/analyzer/lib/src/genera...
File pkg/analyzer/lib/src/generated/resolver.dart (right):

https://codereview.chromium.org/1380963004/diff/1/pkg/analyzer/lib/src/genera...
pkg/analyzer/lib/src/generated/resolver.dart:15104: bool isAssignableTo(DartType
fromType, DartType toType) {
think of it as "fromType isAssignableTo toType"

[Leaf, 2015-10-01 22:38:55]

Nasty one.  Without warnings, our assignability is *almost* symmetric!  LGTM

[Jennifer Messerly, 2015-10-01 22:40:54]

Committed patchset #1 (id:1) manually as
a8570d9e7cfe2427a2bd99348b9ae8199a5903ff (presubmit successful).

[Brian Wilkerson, 2015-10-01 23:50:57]

LGTM