Update comments in core lib showing proper factory syntax.

runtime/lib/error.dart

 // Copyright (c) 2011, 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.
 // Errors are created and thrown by DartVM only.
 // Changes here should also be reflected in corelib/error.dart as well
 
 class AssertionError {
   factory AssertionError._uninstantiable() {
     throw const UnsupportedOperationException(
         "AssertionError can only be allocated by the VM");
   }
   static _throwNew(int assertionStart, int assertionEnd)
       native "AssertionError_throwNew";
   String toString() {
-    return "Failed assertion: '$failedAssertion' is not true " +
-        "in $url at line $line, column $column.";
+    return "'$url': Failed assertion: line $line pos $column: " +
+        "'$failedAssertion' is not true.";
   }
   final String failedAssertion;
   final String url;
   final int line;
   final int column;
 }
 
 class TypeError extends AssertionError {
   factory TypeError._uninstantiable() {
     throw const UnsupportedOperationException(
         "TypeError can only be allocated by the VM");
   }
   String toString() {
-    return "Failed type check: type $srcType is not assignable to type " +
-        "$dstType of $dstName in $url at line " +
-        "$line, column $column.";
+    return "'$url': Failed type check: line $line pos $column: " +
+        "type '$srcType' is not assignable to type '$dstType' of '$dstName'.";
   }
   final String srcType;
   final String dstType;
   final String dstName;
 }
 
 class FallThroughError {
   factory FallThroughError._uninstantiable() {
     throw const UnsupportedOperationException(
         "FallThroughError can only be allocated by the VM");
   }
   static _throwNew(int case_clause_pos) native "FallThroughError_throwNew";
   String toString() {
-    return "Switch case fall-through in $url at line $line.";
+    return "'$url': Switch case fall-through at line $line.";
   }
   final String url;
   final int line;
 }
 
 class InternalError {
   const InternalError(this._msg);
   String toString() => "InternalError: '${_msg}'";
   final String _msg;
 }