[dart2wasm] Pass --enable-threads to wasm-opt

With --import-shared-memory we import a memory, which requires
--enable-threads flag to be passed to wasm-opt.

Change-Id: I25198344f8c918cec28b2b22262e2b9d70a137ea
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/436923
Reviewed-by: Alexander Markov <[email protected]>
Commit-Queue: Ömer Ağacan <[email protected]>

Author: osa1

pkg/dartdev/lib/src/commands/compile.dart

@@ -748,6 +748,7 @@ class CompileWasmCommand extends CompileSubcommandCommand {
       --enable-nontrapping-float-to-int
       --enable-sign-ext
       --enable-bulk-memory
+      --enable-threads
 
       --closed-world
       --traps-never-happen