Implement 'as' operator.

lib/compiler/implementation/ssa/nodes.dart

Index: lib/compiler/implementation/ssa/nodes.dart
diff --git a/lib/compiler/implementation/ssa/nodes.dart b/lib/compiler/implementation/ssa/nodes.dart
index 9157093859f7d1bbc32bb8b6036bb8199ea90e6d..138e78ab3e4cf50649fef89e3f1dd26eef3b7edb 100644
--- a/lib/compiler/implementation/ssa/nodes.dart
+++ b/lib/compiler/implementation/ssa/nodes.dart
@@ -1489,8 +1489,6 @@ class HSubtract extends HBinaryArithmetic {
  * An [HSwitch] instruction has one input for the incoming
  * value, and one input per constant that it can switch on.
  * Its block has one successor per constant, and one for the default.
- * If the switch didn't have a default case, the last successor is
- * the join block.
  */
 class HSwitch extends HControlFlow {
   HSwitch(List<HInstruction> inputs) : super(inputs);
@@ -1498,6 +1496,11 @@ class HSwitch extends HControlFlow {
   HConstant constant(int index) => inputs[index + 1];
   HInstruction get expression() => inputs[0];
 
+  /**
+   * Provides the target to jump to if none of the constants match
+   * the expression. If the switc had no default case, this is the
+   * following join-block.
+   */
   HBasicBlock get defaultTarget() => block.successors.last();
 
   accept(HVisitor visitor) => visitor.visitSwitch(this);
@@ -1813,7 +1816,7 @@ class HNot extends HInstruction {
 }
 
 class HParameterValue extends HInstruction {
-  HParameterValue(element) : super(<HInstruction>[]) {
+  HParameterValue(Element element) : super(<HInstruction>[]) {
     sourceElement = element;
   }
 
@@ -2220,16 +2223,28 @@ class HIs extends HInstruction {
 }
 
 class HTypeConversion extends HCheck {
+  static final int UNCHECKED = 0;
+  static final int CHECKED = 1;
+  static final int CAST = 2;
+
   HType type;
-  final bool checked;
+  final int kind;
 
   HTypeConversion(HType this.type,
                   HInstruction input,
-                  [bool this.checked = false])
+                  [this.kind = UNCHECKED])
     : super(<HInstruction>[input]) {
       sourceElement = input.sourceElement;
   }
 
+  bool get checked() => kind != UNCHECKED;
+  /**
+   * Whether to use cast-semantics instead of checked-mode assignment semantics
+   * for the conversion. The difference is that a cast throws on [:null:] and
+   * throws a different exception on a failed match.
+   */
+  bool get isCast() => kind == CAST;
+
   HType get guaranteedType() => type;
 
   accept(HVisitor visitor) => visitor.visitTypeConversion(this);