[Asm.js] Prevents assignments to immutables.

src/asmjs/asm-typer.cc

Index: src/asmjs/asm-typer.cc
diff --git a/src/asmjs/asm-typer.cc b/src/asmjs/asm-typer.cc
index 4415c30ac6a32b3c6ea59981e7aaa59d11a8f1bc..0c05ab656e133c74f285f6070e3cecb0887a8ad2 100644
--- a/src/asmjs/asm-typer.cc
+++ b/src/asmjs/asm-typer.cc
@@ -1744,7 +1744,10 @@ AsmType* AsmTyper::ValidateAssignmentExpression(Assignment* assignment) {
       return value_type;
     }
 
-    DCHECK(target_info->type() != AsmType::None());
+    if (!target_info->IsMutable()) {
+      FAIL(assignment, "Can't assign to immutable symbol.");
+    }
+
     if (!value_type->IsA(target_info->type())) {
       FAIL(assignment, "Type mismatch in assignment.");
     }