Add support for rethrow and start treating throw <expr> as an expression.

sdk/lib/_internal/compiler/implementation/scanner/listener.dart

Index: sdk/lib/_internal/compiler/implementation/scanner/listener.dart
diff --git a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
index e535f62599ec4a49f1bc805bdffb21cbb8197189..74a78c239399c21f4f1d8cbb1df3255a1dcb03fd 100644
--- a/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
+++ b/sdk/lib/_internal/compiler/implementation/scanner/listener.dart
@@ -339,10 +339,13 @@ class Listener {
   void endSwitchBlock(int caseCount, Token beginToken, Token endToken) {
   }
 
-  void beginThrowStatement(Token token) {
+  void beginThrowExpression(Token token) {
   }
 
-  void endThrowStatement(Token throwToken, Token endToken) {
+  void endThrowExpression(Token throwToken, Token endToken) {
+  }
+
+  void beginRethrowStatement(Token token) {
   }
 
   void endRethrowStatement(Token throwToken, Token endToken) {
@@ -1580,13 +1583,13 @@ class NodeListener extends ElementListener {
     pushNode(new Block(makeNodeList(count, beginToken, endToken, null)));
   }
 
-  void endThrowStatement(Token throwToken, Token endToken) {
+  void endThrowExpression(Token throwToken, Token endToken) {
     Expression expression = popNode();
     pushNode(new Throw(expression, throwToken, endToken));
   }
 
   void endRethrowStatement(Token throwToken, Token endToken) {
-    pushNode(new Throw(null, throwToken, endToken));
+    pushNode(new Rethrow(throwToken, endToken));
   }
 
   void handleUnaryPrefixExpression(Token token) {