Search all the build configurations for sky_snapshot

Previously, we assumed the first build configuration would have one. Now we
keep looking until we find one. Also, re-ordered the configurations so that
you'll get the Android one if you have both, which is probably what you would
expect.

Fixes #100

packages/flutter_tools/lib/src/commands/flutter_command_runner.dart

@@ -195,19 +195,19 @@ class FlutterCommandRunner extends CommandRunner {
         configs.add(new BuildConfiguration.local(
           type: BuildType.debug,
           hostPlatform: hostPlatform,
-          targetPlatform: hostPlatformAsTarget,
+          targetPlatform: TargetPlatform.android,
           enginePath: enginePath,
-          buildPath: globalResults['host-debug-build-path'],
-          testable: true
+          buildPath: globalResults['android-debug-build-path'],
+          deviceId: globalResults['android-device-id']
         ));
 
         configs.add(new BuildConfiguration.local(
           type: BuildType.debug,
           hostPlatform: hostPlatform,
-          targetPlatform: TargetPlatform.android,
+          targetPlatform: hostPlatformAsTarget,
           enginePath: enginePath,
-          buildPath: globalResults['android-debug-build-path'],
-          deviceId: globalResults['android-device-id']
+          buildPath: globalResults['host-debug-build-path'],
+          testable: true
         ));
 
         if (Platform.isMacOS) {
@@ -233,19 +233,19 @@ class FlutterCommandRunner extends CommandRunner {
         configs.add(new BuildConfiguration.local(
           type: BuildType.release,
           hostPlatform: hostPlatform,
-          targetPlatform: hostPlatformAsTarget,
+          targetPlatform: TargetPlatform.android,
           enginePath: enginePath,
-          buildPath: globalResults['host-release-build-path'],
-          testable: true
+          buildPath: globalResults['android-release-build-path'],
+          deviceId: globalResults['android-device-id']
         ));
 
         configs.add(new BuildConfiguration.local(
           type: BuildType.release,
           hostPlatform: hostPlatform,
-          targetPlatform: TargetPlatform.android,
+          targetPlatform: hostPlatformAsTarget,
           enginePath: enginePath,
-          buildPath: globalResults['android-release-build-path'],
-          deviceId: globalResults['android-device-id']
+          buildPath: globalResults['host-release-build-path'],
+          testable: true
         ));
 
         if (Platform.isMacOS) {

packages/flutter_tools/lib/src/toolchain.dart

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:io';
 
 import 'package:path/path.dart' as path;
 
@@ -29,8 +30,12 @@ class Compiler {
 }
 
 Future<String> _getCompilerPath(BuildConfiguration config) async {
-  if (config.type != BuildType.prebuilt)
-    return path.join(config.buildDir, 'clang_x64', 'sky_snapshot');
+  if (config.type != BuildType.prebuilt) {
+    String compilerPath = path.join(config.buildDir, 'clang_x64', 'sky_snapshot');
+    if (FileSystemEntity.isFileSync(compilerPath))
+      return compilerPath;
+    return null;
+  }
   Artifact artifact = ArtifactStore.getArtifact(
     type: ArtifactType.snapshot, hostPlatform: config.hostPlatform);
   return await ArtifactStore.getPath(artifact);
@@ -42,8 +47,11 @@ class Toolchain {
   final Compiler compiler;
 
   static Future<Toolchain> forConfigs(List<BuildConfiguration> configs) async {
-    // TODO(abarth): Shouldn't we consider all the configs?
-    String compilerPath = await _getCompilerPath(configs.first);
-    return new Toolchain(compiler: new Compiler(compilerPath));
+    for (BuildConfiguration config in configs) {
+      String compilerPath = await _getCompilerPath(config);
+      if (compilerPath != null)
+        return new Toolchain(compiler: new Compiler(compilerPath));
+    }
+    return null;
   }
 }