fix #620, infer the input files from sources

tool/build_test_pkgs.sh

+#!/bin/bash
+set -e # bail on error
+
+cd $( dirname "${BASH_SOURCE[0]}" )/..
+
+mkdir -p gen/codegen_output/pkg/
+
+SDK=--dart-sdk-summary=lib/runtime/dart_sdk.sum
+
+./bin/dartdevc.dart $SDK -o gen/codegen_output/pkg/expect.js \
+    --url-mapping=package:expect/expect.dart,test/codegen/expect.dart \
+    package:expect/expect.dart
+
+./bin/dartdevc.dart $SDK  -o gen/codegen_output/pkg/async_helper.js \
+    --url-mapping=package:async_helper/async_helper.dart,test/codegen/async_helper.dart \
+    package:async_helper/async_helper.dart
+
+./bin/dartdevc.dart $SDK  -o gen/codegen_output/pkg/js.js \
+    package:js/js.dart
+
+./bin/dartdevc.dart $SDK  -o gen/codegen_output/pkg/matcher.js \
+    package:matcher/matcher.dart
+
+./bin/dartdevc.dart $SDK  -o gen/codegen_output/pkg/stack_trace.js \
+    package:stack_trace/stack_trace.dart
+
+./bin/dartdevc.dart $SDK  -o gen/codegen_output/pkg/path.js \
+    package:path/path.dart
+
+./bin/dartdevc.dart $SDK  -o gen/codegen_output/pkg/unittest.js \
+    package:unittest/unittest.dart \
+    package:unittest/html_config.dart \
+    package:unittest/html_individual_config.dart \
+    package:unittest/html_enhanced_config.dart