Update almost all tests (except co19) to use the new try-catch syntax.

tests/language/external_test.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.
 
 class Foo {
   var x;
   f() {}
 
   external var x01;  /// 01: compile-time error
   external int x02;  /// 02: compile-time error
@@ skipping 14 matching lines @@
 
 external int t06(int i) { }  /// 30: compile-time error
 external int t07(int i) => i + 1;  /// 31: compile-time error
 
 main() {
 
   // Try calling an unpatched external function.
   var foo = new Foo();                                /// 10: continued
   try {                                               /// 10: continued
     foo.f05();                                        /// 10: continued
-  } catch (String exc) {                              /// 10: continued
+  } on String catch (exc) {                           /// 10: continued
     if (exc == "External implementation missing.") {  /// 10: continued
       throw exc;                                      /// 10: continued
     }                                                 /// 10: continued
   }                                                   /// 10: continued
 
   try {                                               /// 13: continued
     Foo.f13();                                        /// 13: continued
-  } catch (String exc) {                              /// 13: continued
+  } on String catch (exc) {                           /// 13: continued
     if (exc == "External implementation missing.") {  /// 13: continued
       throw exc;                                      /// 13: continued
     }                                                 /// 13: continued
   }                                                   /// 13: continued
 
   // Try calling an unpatched external constructor.
   try {                                               /// 20: continued
     var foo = new Foo.n09();                          /// 20: continued
-  } catch (String exc) {                              /// 20: continued
+  } on String catch (exc) {                           /// 20: continued
     if (exc == "External implementation missing.") {  /// 20: continued
       throw exc;                                      /// 20: continued
     }                                                 /// 20: continued
   }                                                   /// 20: continued
 }