Fix Smi::IsValid assert in StringCharCodeAt deferred code.

src/arm/lithium-codegen-arm.cc

Index: src/arm/lithium-codegen-arm.cc
diff --git a/src/arm/lithium-codegen-arm.cc b/src/arm/lithium-codegen-arm.cc
index 23bfb2144bc658088a111f6e0c235c4c123a3b1d..d9ae6527c0d6348880a551280fe3bd2dfecbafbf 100644
--- a/src/arm/lithium-codegen-arm.cc
+++ b/src/arm/lithium-codegen-arm.cc
@@ -2887,9 +2887,16 @@ void LCodeGen::DoDeferredStringCharCodeAt(LStringCharCodeAt* instr) {
 
   __ PushSafepointRegisters();
   __ push(string);
-  // Push the index as a smi.
+  // Push the index as a smi. It's safe because this instruction must
+  // be dominated by a bounds check and because of the assert.
+  STATIC_ASSERT(String::kMaxLength <= Smi::kMaxValue);
   if (instr->index()->IsConstantOperand()) {
     int const_index = ToInteger32(LConstantOperand::cast(instr->index()));
+    if (!Smi::IsValid(const_index)) {
+      // Guaranteed to be out of bounds, so we can use "-1" to have a
+      // valid smi.
+      const_index = -1;
+    }
     __ mov(scratch, Operand(Smi::FromInt(const_index)));
     __ push(scratch);
   } else {