Throw an exception when invalid source file location is encountered.

lib/compiler/implementation/source_map_builder.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.
 
 #library('source_map_builder');
 
 #import('dart:json');
 
 #import('scanner/scannerlib.dart');
 #import('source_file.dart');
@@ skipping 137 matching lines @@
 
   SourceMapEntry(this.sourceLocation, this.targetOffset);
 }
 
 class SourceFileLocation {
   SourceFile sourceFile;
   Token token;
   int line;
 
   SourceFileLocation(this.sourceFile, this.token) {
-    assert(token.charOffset < sourceFile.text.length);
+    assert(isValid());
   }
 
   String getSourceUrl() => sourceFile.filename;
 
   int getLine() {
     if (line == null) line = sourceFile.getLine(token.charOffset);
     return line;
   }
 
   int getColumn() => sourceFile.getColumn(getLine(), token.charOffset);
 
   String getSourceName() {
     if (token.isIdentifier()) return token.slowToString();
     return null;
   }
+
+  bool isValid() => token.charOffset < sourceFile.text.length;
 }