Paths in source map should be relative.

This makes the paths in the source map relative to the dist/
directory so it can be used on a web server correctly.

Closes #323

Change-Id: Ib69ff2e1393548a3ebd1d67520422173a5827704

build/build.py

@@ -271,7 +271,8 @@ def buildLibrary(self, name, rebuild):
           print 'No changes detected, not building.  Use --force to override.'
           return True
 
-    opts = ['--create_source_map', resultMap, '--js_output_file', resultDebug]
+    opts = ['--create_source_map', resultMap, '--js_output_file', resultDebug,
+            '--source_map_location_mapping', sourceBase + '|..']
     if not self.buildRaw(opts):
       return False