Automatically generate externs.js for a format (idl, json) supported by tools/json_schema_compiler.

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 7a2e4dd6f1593ed3a4ae95b03631d3a483ffd5c0..2efaee917179ac5a5f022b5ce72c4e4f70bd0d0a 100755
--- a/tools/json_schema_compiler/compiler.py
+++ b/tools/json_schema_compiler/compiler.py
@@ -25,6 +25,7 @@ from cpp_bundle_generator import CppBundleGenerator
 from cpp_generator import CppGenerator
 from cpp_type_generator import CppTypeGenerator
 from dart_generator import DartGenerator
+from externs_js_generator import ExternsJsGenerator
 import json_schema
 from cpp_namespace_environment import CppNamespaceEnvironment
 from model import Model
@@ -32,7 +33,8 @@ from schema_loader import SchemaLoader
 
 # Names of supported code generators, as specified on the command-line.
 # First is default.
-GENERATORS = ['cpp', 'cpp-bundle-registration', 'cpp-bundle-schema', 'dart']
+GENERATORS = [
+    'cpp', 'cpp-bundle-registration', 'cpp-bundle-schema', 'dart', 'externs-js']
 
 def GenerateSchema(generator_name,
                    file_paths,
@@ -122,6 +124,8 @@ def GenerateSchema(generator_name,
       ('%s.dart' % namespace.unix_name, DartGenerator(
           dart_overrides_dir))
     ]
+  elif generator_name == 'externs-js':
+    generators = [('externs.js', ExternsJsGenerator())]
   else:
     raise Exception('Unrecognised generator %s' % generator)