Make Exception a class, not an interface, and remove the const constructor.

sdk/lib/core/errors.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 Error {
   const Error();
+
+  /**
+   * Safely convert a value to a [String] description.
+   *
+   * The conversion is guaranteed to not throw, so it won't use the object's
+   * toString method.
+   */
+  static String safeToString(Object object) {
+    if (object is int || object is double || object is bool || null == object) {
+      return object.toString();
+    }
+    if (object is String) {
+      // TODO(ahe): Remove backslash when http://dartbug.com/4995 is fixed.
+      const backslash = '\\';
+      String escaped = object
+        .replaceAll('$backslash', '$backslash$backslash')
+        .replaceAll('\n', '${backslash}n')
+        .replaceAll('\r', '${backslash}r')
+        .replaceAll('"',  '$backslash"');
+      return '"$escaped"';
+    }
+    return _objectToString(object);
+  }
+
+  external static String _objectToString(Object object);
 }
 
 /**
  * Error thrown by the runtime system when an assert statement fails.
  */
 class AssertionError implements Error {
 }
 
 /**
  * Error thrown by the runtime system when a type assertion fails.
@@ skipping 96 matching lines @@
       : this._existingArgumentNames = existingArgumentNames;
 
   String toString() {
     StringBuffer sb = new StringBuffer();
     int i = 0;
     if (_arguments != null) {
       for (; i < _arguments.length; i++) {
         if (i > 0) {
           sb.add(", ");
         }
-        sb.add(safeToString(_arguments[i]));
+        sb.add(Error.safeToString(_arguments[i]));
       }
     }
     if (_namedArguments != null) {
       _namedArguments.forEach((String key, var value) {
         if (i > 0) {
           sb.add(", ");
         }
         sb.add(key);
         sb.add(": ");
-        sb.add(safeToString(value));
+        sb.add(Error.safeToString(value));
         i++;
       });
     }
     if (_existingArgumentNames == null) {
       return "NoSuchMethodError : method not found: '$_memberName'\n"
-          "Receiver: ${safeToString(_receiver)}\n"
+          "Receiver: ${Error.safeToString(_receiver)}\n"
           "Arguments: [$sb]";
     } else {
       String actualParameters = sb.toString();
       sb = new StringBuffer();
       for (int i = 0; i < _existingArgumentNames.length; i++) {
         if (i > 0) {
           sb.add(", ");
         }
         sb.add(_existingArgumentNames[i]);
       }
       String formalParameters = sb.toString();
       return "NoSuchMethodError: incorrect number of arguments passed to "
           "method named '$_memberName'\n"
-          "Receiver: ${safeToString(_receiver)}\n"
+          "Receiver: ${Error.safeToString(_receiver)}\n"
           "Tried calling: $_memberName($actualParameters)\n"
           "Found: $_memberName($formalParameters)";
     }
   }
-
-  static String safeToString(Object object) {
-    if (object is int || object is double || object is bool || null == object) {
-      return object.toString();
-    }
-    if (object is String) {
-      // TODO(ahe): Remove backslash when http://dartbug.com/4995 is fixed.
-      const backslash = '\\';
-      String escaped = object
-        .replaceAll('$backslash', '$backslash$backslash')
-        .replaceAll('\n', '${backslash}n')
-        .replaceAll('\r', '${backslash}r')
-        .replaceAll('"',  '$backslash"');
-      return '"$escaped"';
-    }
-    return _objectToString(object);
-  }
-
-  external static String _objectToString(Object object);
 }
 
 
 /**
  * The operation was not allowed by the object.
  *
  * This [Error] is thrown when a class cannot implement
  * one of the methods in its signature.
  */
 class UnsupportedError implements Error {
@@ skipping 37 matching lines @@
 
 class OutOfMemoryError implements Error {
   const OutOfMemoryError();
   String toString() => "Out of Memory";
 }
 
 class StackOverflowError implements Error {
   const StackOverflowError();
   String toString() => "Stack Overflow";
 }