dart2js cps: Add CharCodeAt builtin.

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

Index: pkg/compiler/lib/src/cps_ir/type_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/type_propagation.dart b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
index 61c92001b092648d45cca2ac759f1679e633227b..aa8c972bdda6e298e288589a8a6a6b6ca5837dd4 100644
--- a/pkg/compiler/lib/src/cps_ir/type_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/type_propagation.dart
@@ -452,6 +452,10 @@ class ConstantPropagationLattice {
     return folded ?? closedOnUint(left, right) ?? closedOnInt(left, right);
   }
 
+  AbstractValue codeUnitAtSpecial(AbstractValue left, AbstractValue right) {
+    return foldBinary(constantSystem.codeUnitAt, left, right);
+  }
+
   AbstractValue equalSpecial(AbstractValue left, AbstractValue right) {
     AbstractValue folded = foldBinary(constantSystem.equal, left, right);
     if (folded != null) return folded;
@@ -1141,6 +1145,21 @@ class TransformingVisitor extends DeepRecursiveVisitor {
                 replaceWithBinary(BuiltinOperator.NumRemainder, receiver, arg);
           }
         }
+      } else if (name == 'codeUnitAt') {
+        if (node.arguments.length == 2) {
+          Primitive index = node.dartArgument(0);
+          if (lattice.isDefinitelyString(receiverValue) &&
+              lattice.isDefinitelyInt(getValue(index))) {
+            SourceInformation sourceInfo = node.sourceInformation;
+            CpsFragment cps = makeBoundsCheck(receiver, index, sourceInfo);
+            ApplyBuiltinOperator get =
+                cps.applyBuiltin(BuiltinOperator.CharCodeAt,
+                                 <Primitive>[receiver, index]);
+            node.replaceUsesWith(get);
+            get.hint = node.hint;
+            return cps;
+          }
+        }
       }
     }
     return null;
@@ -2518,15 +2537,34 @@ class TypePropagationVisitor implements Visitor {
       return;  // And come back later.
     }
 
-    // Constant fold known length of containers.
-    if (node.selector == Selectors.length) {
-      AbstractValue object = getValue(node.dartReceiver);
-      if (typeSystem.isDefinitelyIndexable(object.type, allowNull: true)) {
-        AbstractValue length = lattice.lengthSpecial(object);
-        if (length != null) {
-          setResult(node, length, canReplace: !object.isNullable);
+    if (node.selector.isGetter) {
+      // Constant fold known length of containers.
+      if (node.selector == Selectors.length) {
+        AbstractValue object = getValue(node.dartReceiver);
+        if (typeSystem.isDefinitelyIndexable(object.type, allowNull: true)) {
+          AbstractValue length = lattice.lengthSpecial(object);
+          if (length != null) {
+            setResult(node, length, canReplace: !object.isNullable);
+          }
         }
       }
+      setResult(node, lattice.getInvokeReturnType(node.selector, node.mask));
+      return;
+    }
+
+    if (node.selector.isCall) {
+      AbstractValue result;
+      if (node.selector == Selectors.codeUnitAt) {
+        AbstractValue object = getValue(node.dartReceiver);
+        AbstractValue right = getValue(node.dartArgument(0));
+        result = lattice.codeUnitAtSpecial(object, right);
+      }
+      if (result == null) {
+        setResult(node, lattice.getInvokeReturnType(node.selector, node.mask));
+      } else {
+        setResult(node, result, canReplace: true);
+      }
+      return;
     }
 
     if (!node.selector.isOperator) {
@@ -2619,6 +2657,10 @@ class TypePropagationVisitor implements Visitor {
         }
         break;
 
+      case BuiltinOperator.CharCodeAt:
+        binaryOp(lattice.codeUnitAtSpecial, typeSystem.uint31Type);
+        break;
+
       case BuiltinOperator.Identical:
       case BuiltinOperator.StrictEq:
       case BuiltinOperator.LooseEq: