In Scala.js mode, compile all lazy vals as thread-unsafe.

Since JS is a single-threaded environment, there is no point going
through all the machinery of thread-safe lazy vals.

This fixes lazy vals for JS, which would previously fail to link
because of the dependency on `Unsafe`, which is not supported in
Scala.js.

compiler/src/dotty/tools/dotc/transform/LazyVals.scala

@@ -76,8 +76,8 @@ class LazyVals extends MiniPhase with IdentityDenotTransformer {
       if (isField) {
         if (sym.isAllOf(SyntheticModule))
           transformSyntheticModule(tree)
-        else if (sym.isThreadUnsafe) {
-          if (sym.is(Module)) {
+        else if (sym.isThreadUnsafe || ctx.settings.scalajs.value) {
+          if (sym.is(Module) && !ctx.settings.scalajs.value) {
             ctx.error(em"@threadUnsafe is only supported on lazy vals", sym.sourcePos)
             transformMemberDefThreadSafe(tree)
           }
@@ -453,6 +453,3 @@ object LazyVals {
     val retry: TermName       = "retry".toTermName
   }
 }
-
-
-

project/Build.scala

@@ -937,6 +937,7 @@ object Build {
         val dir = fetchScalaJSSource.value / "test-suite"
         (
           (dir / "shared/src/test/scala/org/scalajs/testsuite/compiler" ** "IntTest.scala").get
+          ++ (dir / "shared/src/test/require-jdk8/org/scalajs/testsuite/javalib/util" ** "Base64Test.scala").get
           ++ (dir / "shared/src/test/scala/org/scalajs/testsuite/utils" ** "*.scala").get
         )
       }