Protect concurrent accesses to JSDefinitions.cache.

Author: sjrd

src/dotty/tools/backend/sjs/JSDefinitions.scala

@@ -2,14 +2,18 @@ package dotty.tools.backend.sjs
 
 import dotty.tools.dotc.core._
 
-import Types._, Contexts._, Symbols._, Denotations._, SymDenotations._, StdNames._, Names._
-import Flags._, Scopes._, Decorators._, NameOps._, Periods._
+import Types._
+import Contexts._
+import Symbols._
+import StdNames._
+import Decorators._
 
 object JSDefinitions {
+  @dotty.tools.sharable
   private val cache = new java.util.WeakHashMap[ContextBase, JSDefinitions]
 
   // TODO Figure out where best to define this
-  def jsdefn(implicit ctx: Context): JSDefinitions = {
+  def jsdefn(implicit ctx: Context): JSDefinitions = cache.synchronized {
     val baseCtx = ctx.base
     val cached = cache.get(baseCtx)
     if (cached != null) {