Implement 'as' operator.

lib/compiler/implementation/ssa/builder.dart

Index: lib/compiler/implementation/ssa/builder.dart
diff --git a/lib/compiler/implementation/ssa/builder.dart b/lib/compiler/implementation/ssa/builder.dart
index 7e4662fa024edf27adba4eeb118a72d8f8962ef6..5f7c6597eb8bf0ad3f98f367c7fb4087173b5b8f 100644
--- a/lib/compiler/implementation/ssa/builder.dart
+++ b/lib/compiler/implementation/ssa/builder.dart
@@ -1073,7 +1073,12 @@ class SsaBuilder extends ResolvedVisitor implements Visitor {
   HInstruction potentiallyCheckType(HInstruction original,
                                     Element sourceElement) {
     if (!compiler.enableTypeAssertions) return original;
+    return convertType(original, sourceElement, HTypeConversion.CHECKED);
+  }
 
+  HInstruction convertType(HInstruction original,
+                           Element sourceElement,
+                           int kind) {
     Type type = sourceElement.computeType(compiler);
     if (type === null) return original;
     if (type.element === compiler.dynamicClass) return original;
@@ -1088,7 +1093,7 @@ class SsaBuilder extends ResolvedVisitor implements Visitor {
     }
 
     HInstruction instruction =
-        new HTypeConversion(convertedType, original, true);
+        new HTypeConversion(convertedType, original, kind);
     add(instruction);
     return instruction;
   }
@@ -1959,6 +1964,15 @@ class SsaBuilder extends ResolvedVisitor implements Visitor {
         }
         push(instruction);
       }
+    } else if (const SourceString("as") == op.source) {
+      visit(node.receiver);
+      HInstruction expression = pop();
+      Node argument = node.arguments.head;
+      TypeAnnotation typeAnnotation = argument.asTypeAnnotation();
+      Type type = elements.getType(typeAnnotation);
+      HInstruction converted =
+          convertType(expression, type.element, HTypeConversion.CAST);
+      stack.add(converted);
     } else {
       visit(node.receiver);
       visit(node.argumentsNode);