MIPS: port Parse harmony let declarations.

src/mips/full-codegen-mips.cc

Index: src/mips/full-codegen-mips.cc
diff --git a/src/mips/full-codegen-mips.cc b/src/mips/full-codegen-mips.cc
index d8909c9ddd4c536affd089f16c5e080986dcf8c9..856f5179a74f2222ed8a9df98d625c901fc7b72c 100644
--- a/src/mips/full-codegen-mips.cc
+++ b/src/mips/full-codegen-mips.cc
@@ -747,9 +747,9 @@ void FullCodeGenerator::EmitDeclaration(Variable* variable,
         __ li(a2, Operand(variable->name()));
         // Declaration nodes are always introduced in one of two modes.
         ASSERT(mode == Variable::VAR ||
-               mode == Variable::CONST);
-        PropertyAttributes attr =
-            (mode == Variable::VAR) ? NONE : READ_ONLY;
+               mode == Variable::CONST ||
+               mode == Variable::LET);
+        PropertyAttributes attr = (mode == Variable::CONST) ? READ_ONLY : NONE;
         __ li(a1, Operand(Smi::FromInt(attr)));
         // Push initial value, if any.
         // Note: For variables we must not push an initial value (such as