[strong] Allow mutually recursive classes.

src/scopes.cc

Index: src/scopes.cc
diff --git a/src/scopes.cc b/src/scopes.cc
index b796452e00589b17b837a0eb6b5224885d9c389b..364f2f1cf7ce88ca08e207172c1cba1613dbb3f7 100644
--- a/src/scopes.cc
+++ b/src/scopes.cc
@@ -1138,6 +1138,30 @@ bool Scope::CheckStrongModeDeclaration(VariableProxy* proxy, Variable* var) {
     scope = scope->outer_scope();
   }
 
+  // Allow references from methods to classes declared later, if we detect no
+  // problematic dependency cycles.
+
+  if (ClassVariableForMethod() && var->is_class()) {
+    // A method is referring to some other class, possibly declared
+    // later. Referring to a class declared earlier is always OK and covered by
+    // the code outside this if. Here we only need to allow special cases for
+    // referring to a class which is declared later.
+
+    // Referring to a class C declared later is OK under the following
+    // circumstances:
+
+    // 1. The class declarations are in a consecutive group with no other
+    // declarations or statements in between, and
+
+    // 2. There is no dependency cycle where the first edge is an initialization
+    // time dependency (computed property name or extends clause) from C to
+    // something that depends on this class directly or transitively.
+
+    // TODO(marja,rossberg): implement these checks. Here we undershoot the
+    // target and allow referring to any class.
+    return true;
+  }
+
   // If both the use and the declaration are inside an eval scope (possibly
   // indirectly), or one of them is, we need to check whether they are inside
   // the same eval scope or different ones.