Report errors with return (e.g., when missing) on Mac OS X.

tools/gyp/configurations_xcode.gypi

 # Copyright (c) 2011, 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.
 
 {
   'target_defaults': {
     'configurations': {
       'Dart_Base': {
         'xcode_settings': {
           # To switch to the LLVM based backend change the two lines below.
           #'GCC_VERSION': 'com.apple.compilers.llvmgcc42',
           'GCC_VERSION': '4.2',
           'GCC_C_LANGUAGE_STANDARD': 'ansi',
           'GCC_ENABLE_CPP_EXCEPTIONS': 'NO', # -fno-exceptions
           'GCC_ENABLE_CPP_RTTI': 'NO', # -fno-rtti
           'GCC_DEBUGGING_SYMBOLS': 'default', # -g
           'GCC_GENERATE_DEBUGGING_SYMBOLS': 'YES', # Do not strip symbols
           'GCC_SYMBOLS_PRIVATE_EXTERN': 'YES', # -fvisibility=hidden
           'GCC_INLINES_ARE_PRIVATE_EXTERN': 'YES', # -fvisibility-inlines-hidden
           'GCC_WARN_NON_VIRTUAL_DESTRUCTOR': 'YES', # -Wnon-virtual-dtor
           # TODO(v8-team): Fix V8 build.
           #'GCC_WARN_HIDDEN_VIRTUAL_FUNCTIONS': 'YES', # -Woverloaded-virtual
           'GCC_TREAT_WARNINGS_AS_ERRORS': 'YES', # -Werror
           'WARNING_CFLAGS': [
             '<@(common_gcc_warning_flags)',
             '-Wtrigraphs', # Disable Xcode default.
+            '-Wreturn-type',
+            '-Werror=return-type',
           ],
 
           # Generate PIC code as Chrome is switching to this.
           'GCC_DYNAMIC_NO_PIC': 'NO',
 
           # When searching for header files, do not search
           # subdirectories. Without this option, vm/assert.h conflicts
           # with the system header assert.h. Apple also recommend
           # setting this to NO.
           'ALWAYS_SEARCH_USER_PATHS': 'NO',
@@ skipping 33 matching lines @@
       },
 
       'Dart_Release': {
         'xcode_settings': {
           'GCC_OPTIMIZATION_LEVEL': '3',
         },
       },
     },
   },
 }