Generate externs automatically from json/idl files.

tools/json_schema_compiler/compiler.py

Index: tools/json_schema_compiler/compiler.py
diff --git a/tools/json_schema_compiler/compiler.py b/tools/json_schema_compiler/compiler.py
index acf64acd91d0226ab2959ba1faef20b12c75722e..4fee004505528852742072847d03da43eec245f0 100755
--- a/tools/json_schema_compiler/compiler.py
+++ b/tools/json_schema_compiler/compiler.py
@@ -24,13 +24,14 @@ from cpp_bundle_generator import CppBundleGenerator
 from cpp_generator import CppGenerator
 from cpp_type_generator import CppTypeGenerator
 from dart_generator import DartGenerator
+from js_externs_generator import JsExternsGenerator
 import json_schema
 from model import Model, UnixName
 from schema_loader import SchemaLoader
 
 # Names of supported code generators, as specified on the command-line.
 # First is default.
-GENERATORS = ['cpp', 'cpp-bundle', 'dart']
+GENERATORS = ['cpp', 'cpp-bundle', 'dart', 'externs']
 
 def GenerateSchema(generator,
                    filenames,
@@ -110,6 +111,10 @@ def GenerateSchema(generator,
       ('%s.dart' % namespace.unix_name, DartGenerator(
           dart_overrides_dir))
     ]
+  elif generator == 'externs':
+    generators = [
+      ('%s_externs.js' % namespace.unix_name, JsExternsGenerator())
+    ]
   else:
     raise Exception('Unrecognised generator %s' % generator)