dart2js cps: Logical rewriter should treat Throw as a terminator.

Because of this bug, the tree IR had a tendency to put throw
statements at the bottom of a method, even if it meant inserting
an explicit negation in a branch.

BUG=
R=sra@google.com

Review URL: https://codereview.chromium.org//1368963002 .

pkg/compiler/lib/src/tree_ir/optimization/logical_rewriter.dart

@@ -95,7 +95,8 @@ class LogicalRewriter extends RecursiveTransformer
 
   bool isTerminator(Statement node) {
     return (node is Jump || node is Return) && !isFallthrough(node) ||
-           (node is ExpressionStatement && node.next is Unreachable);
+           (node is ExpressionStatement && node.next is Unreachable) ||
+           node is Throw;
   }
 
   Statement visitIf(If node) {