[Asm.js] Prevents assignments to immutables.

Adds a check so that module validation fails if there are any
assignments to immutable identifiers (e.g., module name, module
parameters, stdlib imports.)

BUG= https://bugs.chromium.org/p/chromium/issues/detail?id=640194

Review-Url: https://codereview.chromium.org/2268363002
Cr-Commit-Position: refs/heads/master@{#38838}

src/asmjs/asm-typer.cc

@@ -1744,7 +1744,11 @@ 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.");
+    }
+
+    DCHECK_NE(AsmType::None(), target_info->type());
     if (!value_type->IsA(target_info->type())) {
       FAIL(assignment, "Type mismatch in assignment.");
     }

test/cctest/asmjs/test-asm-typer.cc

@@ -1941,4 +1941,63 @@ TEST(B63099) {
   }
 }
 
+// This issue was triggered because assignments to immutable symbols (e.g., the
+// module's name, or any of the asm.js' module parameters) was not being
+// handled.
+TEST(B640194) {
+  const char* kTests[] = {
+      "function asm() {\n"
+      "  'use asm';\n"
+      "  function f() {\n"
+      "    asm = 0;\n"
+      "  }\n"
+      "  return f;\n"
+      "}",
+      "function asm(stdlib) {\n"
+      "  'use asm';\n"
+      "  function f() {\n"
+      "    stdlib = 0;\n"
+      "  }\n"
+      "  return f;\n"
+      "}",
+      "function asm(stdlib, foreign) {\n"
+      "  'use asm';\n"
+      "  function f() {\n"
+      "    foreign = 0;\n"
+      "  }\n"
+      "  return f;\n"
+      "}",
+      "function asm(stdlib, foreign, heap) {\n"
+      "  'use asm';\n"
+      "  function f() {\n"
+      "    heap = 0;\n"
+      "  }\n"
+      "  return f;\n"
+      "}",
+      "function asm(stdlib, foreign, heap) {\n"
+      "  'use asm';\n"
+      "  var f = stdlib.Math.fround;\n"
+      "  function f() {\n"
+      "    f = 0;\n"
+      "  }\n"
+      "  return f;\n"
+      "}",
+      "function asm(stdlib, foreign, heap) {\n"
+      "  'use asm';\n"
+      "  var E = stdlib.Math.E;\n"
+      "  function f() {\n"
+      "    E = 0;\n"
+      "  }\n"
+      "  return f;\n"
+      "}",
+  };
+  for (size_t ii = 0; ii < arraysize(kTests); ++ii) {
+    if (!ValidationOf(Module(kTests[ii]))
+             ->FailsWithMessage("Can't assign to immutable symbol")) {
+      std::cerr << "Test:\n" << kTests[ii];
+      CHECK(false);
+    }
+  }
+}
+
 }  // namespace