make build_sdk more incremental

tool/build_sdk.sh

Index: tool/build_sdk.sh
diff --git a/tool/build_sdk.sh b/tool/build_sdk.sh
index 9e3dde2f09554989d745ef600edfeeb07356b0d2..ccb064e23322ca99a219f0db13330bc3b1b15b05 100755
--- a/tool/build_sdk.sh
+++ b/tool/build_sdk.sh
@@ -4,24 +4,11 @@ set -e
 cd $( dirname "${BASH_SOURCE[0]}" )/..
 
 echo "*** Patching SDK"
-rm -r tool/generated_sdk || true
 dart -c tool/patch_sdk.dart tool/input_sdk tool/generated_sdk
 
 echo "*** Compiling SDK to JavaScript"
-if [[ -d lib/runtime/dart ]] ; then
-  rm -r lib/runtime/dart
-fi
 
-# TODO(jmesserly): this builds dart:js, which pulls in dart:core and many others
-# transitively. Ideally we could pass them explicitly, though:
-# https://github.com/dart-lang/dev_compiler/issues/219
 dart -c bin/devc.dart --no-source-maps --arrow-fn-bind-this --sdk-check \
     --force-compile -l warning --dart-sdk tool/generated_sdk -o lib/runtime/ \
     dart:js dart:mirrors \
     > tool/sdk_expected_errors.txt || true
-
-if [[ ! -f lib/runtime/dart/core.js ]] ; then
-    echo 'core.js not found, assuming build failed.'
-    echo './tool/build_sdk.sh can be run to reproduce this.'
-    exit 1
-fi