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

sdk/lib/_internal/compiler/implementation/tree/unparser.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 part of tree;
 
 String unparse(Node node) {
   Unparser unparser = new Unparser();
   unparser.unparse(node);
   return unparser.result;
@@ skipping 248 matching lines @@
     if (node.nodes != null) {
       unparseNodeListFrom(node, node.nodes);
     }
     if (node.endToken != null) add(node.endToken.value);
   }
 
   visitOperator(Operator node) {
     visitIdentifier(node);
   }
 
+  visitRethrow(Rethrow node) {
+    sb.write('rethrow;');
+  }
+
   visitReturn(Return node) {
     if (node.isRedirectingFactoryBody) {
       sb.write(' ');
     }
     add(node.beginToken.value);
     if (node.hasExpression && node.beginToken.stringValue != '=>') {
       sb.write(' ');
     }
     visit(node.expression);
     if (node.endToken != null) add(node.endToken.value);
@@ skipping 50 matching lines @@
       if (!node.isPrefix) {
         visit(node.assignmentOperator);
         if (node.assignmentOperator.source.slowToString() != '=') sb.write(' ');
       }
       visit(node.argumentsNode);
     }
   }
 
   visitThrow(Throw node) {
     add(node.throwToken.value);
-    if (node.expression != null) {
-      sb.write(' ');
-      visit(node.expression);
-    }
-    node.endToken.value.printOn(sb);
+    sb.write(' ');
+    visit(node.expression);
   }
 
   visitTypeAnnotation(TypeAnnotation node) {
     visit(node.typeName);
     visit(node.typeArguments);
   }
 
   visitTypeVariable(TypeVariable node) {
     visit(node.name);
     if (node.bound != null) {
@@ skipping 264 matching lines @@
   }
 
   visitStatement(Statement node) {
     throw 'internal error'; // Should not be called.
   }
 
   visitStringNode(StringNode node) {
     throw 'internal error'; // Should not be called.
   }
 }