Use elements/types.dart in codegen.dart

pkg/compiler/lib/src/ssa/nodes.dart

Index: pkg/compiler/lib/src/ssa/nodes.dart
diff --git a/pkg/compiler/lib/src/ssa/nodes.dart b/pkg/compiler/lib/src/ssa/nodes.dart
index 429178ed8a93b766f1fe0b06cc7dd5fbb39de57f..15283a71190bf59b274668bccebb6e169dbfa319 100644
--- a/pkg/compiler/lib/src/ssa/nodes.dart
+++ b/pkg/compiler/lib/src/ssa/nodes.dart
@@ -3293,7 +3293,7 @@ enum TypeInfoExpressionKind { COMPLETE, INSTANCE }
 /// The INSTANCE form is constructed as a list of [inputs]. This is the same as
 /// the COMPLETE form for the 'thisType', except the root term's type is
 /// missing; this is implicit as the raw type of instance.  The [dartType] of
-/// the INSTANCE form must be the thisType of some class.
+/// the INSTANCE form must be a raw type of some class.

[sra1, 2017/01/04 17:16:03]

Can we avoid this change?
It breaks the invariant that each type variable in the dartType corresponds to
an input.

INSTANCE/COMPLETE is an unfortunate distinction because the *current* reified
representation is 'headless', with the outermost term being implicit.  This will
change if/when we store a complete reified type (e.g see TODO below).

Type-forwarding optimizations rely on being able to call asInstanceOf, which
gives different results for raw- and this- types.

[Johnni Winther, 2017/01/06 10:24:04]

Done.

 ///
 /// We want to remove the constrains on the INSTANCE form. In the meantime we
 /// get by with a tree of TypeExpressions.  Consider: