When targeting iOS sim in preview-dart-2, always compile. (#14059)

Fixes https://github.com/flutter/flutter/issues/14058.

packages/flutter_tools/lib/src/flx.dart

@@ -69,7 +69,7 @@ Future<Null> build({
   }
 
   DevFSContent kernelContent;
-  if (previewDart2) {
+  if (!precompiledSnapshot && previewDart2) {
     final String kernelBinaryFilename = await compile(
       sdkRoot: artifacts.getArtifactPath(Artifact.flutterPatchedSdkPath),
       incrementalCompilerByteStorePath: fs.path.absolute(getIncrementalCompilerByteStoreDirectory()),

packages/flutter_tools/lib/src/ios/simulators.dart

@@ -413,7 +413,11 @@ class IOSSimulator extends Device {
   }
 
   Future<Null> _sideloadUpdatedAssetsForInstalledApplicationBundle(ApplicationPackage app, BuildInfo buildInfo) =>
-      flx.build(precompiledSnapshot: true, previewDart2: buildInfo.previewDart2, strongMode: buildInfo.strongMode);
+      // When running in previewDart2 mode, we still need to run compiler to
+      // produce kernel file for the application.
+      flx.build(precompiledSnapshot: !buildInfo.previewDart2,
+          previewDart2: buildInfo.previewDart2,
+          strongMode: buildInfo.strongMode);
 
   @override
   Future<bool> stopApp(ApplicationPackage app) async {