dart2js cps: Add CharCodeAt builtin.

pkg/compiler/lib/src/cps_ir/builtin_operator.dart

Index: pkg/compiler/lib/src/cps_ir/builtin_operator.dart
diff --git a/pkg/compiler/lib/src/cps_ir/builtin_operator.dart b/pkg/compiler/lib/src/cps_ir/builtin_operator.dart
index 9c1d83e18e93680c448ebec832cb1a2f874fd90b..4fe8e19bd7153cb0fa51de134bf60482ade358e3 100644
--- a/pkg/compiler/lib/src/cps_ir/builtin_operator.dart
+++ b/pkg/compiler/lib/src/cps_ir/builtin_operator.dart
@@ -70,6 +70,10 @@ enum BuiltinOperator {
   /// Returns the empty string if no arguments are given.
   StringConcatenate,
 
+  /// Corresponds to `a.charCodeAt(b)`. `a' must be a String. The index `b` must
+  /// be in range `0 <= b < a.length`.
+  CharCodeAt,

[sra1, 2015/11/23 20:39:32]

Question: will GVN hoist this above the bounds check?
There is no edge from the bounds check to the bounds use to prevent that.

[asgerf, 2015/11/24 11:28:31]

Yes, nice catch. We should disable lifting for these. We could just make it a
BuiltinMethod until we land a better infrastructure to keep track of these
things.

We might want to add "witness" nodes like in the SSA (essentially non-unary
refinements), but unless BCE is going to hoist witness nodes and follow up with
another LICM pass, I think the effect will be same as just disabling lifting for
the nodes that need such witnesses.

+
   /// Returns true if the two arguments are the same value, and that value is
   /// not NaN, or if one argument is +0 and the other is -0.
   ///