Revive throw as an expression but not for rethrow.

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

Index: sdk/lib/_internal/compiler/implementation/tree/nodes.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/tree/nodes.dart	(revision 15894)
+++ sdk/lib/_internal/compiler/implementation/tree/nodes.dart	(working copy)
@@ -72,7 +72,7 @@
   }
   R visitSwitchCase(SwitchCase node) => visitNode(node);
   R visitSwitchStatement(SwitchStatement node) => visitStatement(node);
-  R visitThrow(Throw node) => visitStatement(node);
+  R visitThrow(Throw node) => visitExpression(node);
   R visitTryStatement(TryStatement node) => visitStatement(node);
   R visitTypeAnnotation(TypeAnnotation node) => visitNode(node);
   R visitTypedef(Typedef node) => visitNode(node);
@@ -940,7 +940,7 @@
   Token getEndToken() => endToken;
 }
 
-class Throw extends Statement {
+class Throw extends Expression {
   final Expression expression;
 
   final Token throwToken;