Add additional asserts to ensure we don't create diagnostics with bogus positions.

Author: CyrusNajmabadi

src/compiler/core.ts

@@ -269,8 +269,12 @@ module ts {
 
     export function createFileDiagnostic(file: SourceFile, start: number, length: number, message: DiagnosticMessage, ...args: any[]): Diagnostic;
     export function createFileDiagnostic(file: SourceFile, start: number, length: number, message: DiagnosticMessage): Diagnostic {
+        var end = start + length;
+
         Debug.assert(start >= 0, "start must be non-negative, is " + start);
         Debug.assert(length >= 0, "length must be non-negative, is " + length);
+        Debug.assert(start <= file.text.length, `start must be within the bounds of the file. ${ start } > ${ file.text.length }`);
+        Debug.assert(end <= file.text.length, `end must be the bounds of the file. ${ end } > ${ file.text.length }`);
 
         var text = getLocaleSpecificMessage(message.key);