Make modInv throw Exception on incompatible operands.

sdk/lib/core/exceptions.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.
 
 part of dart.core;
 
 // Exceptions are thrown either by the VM or from Dart code.
 
 /**
  * A marker interface implemented by all core library exceptions.
  *
  * An [Exception] is intended to convey information to the user about a failure,
  * so that the error can be addressed programmatically. It is intended to be
  * caught, and it should contain useful data fields.
  *
  * Creating instances of [Exception] directly with [:new Exception("message"):]
  * is discouraged, and only included as a temporary measure during development,
  * until the actual exceptions used by a library are done.
  */
 abstract class Exception {
-  factory Exception([var message]) => new _ExceptionImplementation(message);
+  factory Exception([var message]) => new _Exception(message);
 }
 
 
 /** Default implementation of [Exception] which carries a message. */
-class _ExceptionImplementation implements Exception {
+class _Exception implements Exception {
   final message;
 
-  _ExceptionImplementation([this.message]);
+  _Exception([this.message]);
 
   String toString() {
     if (message == null) return "Exception";
     return "Exception: $message";
   }
 }
 
 
 /**
  * Exception thrown when a string or some other data does not have an expected
@@ skipping 127 matching lines @@
         end = offset + 36;
         prefix = postfix = "...";
       }
     }
     String slice = source.substring(start, end);
     int markOffset = offset - start + prefix.length;
     return "$report$prefix$slice$postfix\n${" " * markOffset}^\n";
   }
 }
 
+// Exception thrown when doing integer division with a zero divisor.
 class IntegerDivisionByZeroException implements Exception {
   const IntegerDivisionByZeroException();
   String toString() => "IntegerDivisionByZeroException";
 }