Compute default throws behavior from JavaScript.

pkg/js_ast/lib/src/builder.dart

 // Copyright (c) 2013, 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.
 
 // Utilities for building JS ASTs at runtime.  Contains a builder class
 // and a parser that parses part of the language.
 
 part of js_ast;
 
 
@@ skipping 593 matching lines @@
       }
       lastToken = src.substring(lastPosition, position);
     } else {
       // All other tokens handled here.
       int cat = category(src.codeUnitAt(position));
       int newCat;
       do {
         position++;
         if (position == src.length) break;
         int code = src.codeUnitAt(position);
-        // Special code to disallow ! and / in non-first position in token, so
-        // that !! parses as two tokens and != parses as one, while =/ parses
-        // as a an equals token followed by a regexp literal start.
-        newCat = (code == charCodes.$BANG || code == charCodes.$SLASH)
-            ?  NONE
+        // Special code to disallow !, ~ and / in non-first position in token,
+        // so that !! and ~~ parse as two tokens and != parses as one, while =/
+        // parses as a an equals token followed by a regexp literal start.
+        newCat =
+            (code == charCodes.$BANG ||
+             code == charCodes.$SLASH ||
+             code == charCodes.$TILDE)
+            ? NONE
             : category(code);
       } while (!singleCharCategory(cat) &&
                (cat == newCat ||
                 (cat == ALPHA && newCat == NUMERIC) ||    // eg. level42.
                 (cat == NUMERIC && newCat == DOT)));      // eg. 3.1415
       lastCategory = cat;
       lastToken = src.substring(lastPosition, position);
       if (cat == NUMERIC) {
         double.parse(lastToken, (_) {
           error("Unparseable number");
@@ skipping 700 matching lines @@
 
   Catch parseCatch() {
     expectCategory(LPAREN);
     Declaration errorName = parseVariableDeclaration();
     expectCategory(RPAREN);
     expectCategory(LBRACE);
     Block body = parseBlock();
     return new Catch(errorName, body);
   }
 }