Fix cacthing exception object: a null exception is always caught.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 4404)
+++ runtime/vm/parser.cc	(working copy)
@@ -4845,11 +4845,13 @@
   // operator.
   bool catch_seen = false;
   bool generic_catch_seen = false;
+  intptr_t catch_clause_count = 0;
   SequenceNode* catch_handler_list = NULL;
   const intptr_t handler_pos = token_index_;
   OpenBlock();  // Start the catch block sequence.
   current_block_->scope->AddLabel(end_catch_label);
   while (CurrentToken() == Token::kCATCH) {
+    catch_clause_count++;
     catch_seen = true;
     const intptr_t catch_pos = token_index_;
     ConsumeToken();  // Consume the 'catch'.
@@ -4920,10 +4922,22 @@
       }
       AstNode* exception_type = new TypeNode(catch_pos, *exception_param.type);
       AstNode* exception_var = new LoadLocalNode(catch_pos, *catch_excp_var);
-      AstNode* cond_expr = new ComparisonNode(
+      AstNode* type_cond_expr = new ComparisonNode(
           catch_pos, Token::kIS, exception_var, exception_type);
-      current_block_->statements->Add(
-          new IfNode(catch_pos, cond_expr, catch_handler, NULL));
+      if (catch_clause_count == 1) {
+        // Null is also allowed, but check only in the first clause.
+        AstNode* null_literal =
+            new LiteralNode(catch_pos, Instance::ZoneHandle(Instance::null()));
+        AstNode* null_cond_expr = new ComparisonNode(
+            catch_pos, Token::kEQ_STRICT, exception_var, null_literal);
+        AstNode* or_node = new BinaryOpNode(
+            catch_pos, Token::kOR, null_cond_expr, type_cond_expr);
+        current_block_->statements->Add(
+            new IfNode(catch_pos, or_node, catch_handler, NULL));
+      } else {
+        current_block_->statements->Add(
+            new IfNode(catch_pos, type_cond_expr, catch_handler, NULL));
+      }
     } else {
       // No exception type exists in the catch specifier so execute the
       // catch handler code unconditionally.