Bugfix: Make ISO monthCode abstract operations match polyfill. (#1362)

This is subject to further change based on IETF standardization of monthCodes; it is not going to be defined by temporal.

Fixes #1348.

Author: cjtenny

spec/calendar.html

@@ -299,7 +299,11 @@ <h1>ResolveISOMonth ( _fields_ )</h1>
         1. If _monthCode_ is *undefined*, then
           1. If _month_ is *undefined*, throw a *TypeError* exception.
           1. Return _month_.
-        1. Let _numberPart_ be ! ToNumber(_monthCode_).
+        1. Assert: Type(_monthCode_) is String.
+        1. Let _monthLength_ be the length of _monthCode_.
+        1. If _monthLength_ is not 3, throw a *RangeError* exception.
+        1. Let _numberPart_ be the substring of _monthCode_ from 2.
+        1. Set _numberPart_ to ! ToIntegerOrInfinity(_numberPart_).
         1. If _numberPart_ is *NaN*, throw a *RangeError* exception.
         1. If _month_ is not *undefined*, and _month_ ≠ _numberPart_, then
           1. Throw a *RangeError* exception.
@@ -409,7 +413,8 @@ <h1>ISOMonthCode ( _dateOrDateTime_ )</h1>
       <emu-alg>
         1. If _dateOrDateTime_ does not have an [[ISOMonth]] internal slot, then
           1. Set _dateOrDateTime_ to ? ToTemporalDate(_dateOrDateTime_).
-        1. Return ! ES.ToString(_dateOrDateTime_.[[ISOMonth]]).
+        1. Let _monthCode_ be the String representation of _dateOrDateTime_.[[ISOMonth]], formatted as a two-digit decimal number, padded to the left with the code unit 0x0030 (DIGIT ZERO) if necessary.
+        1. Return the string-concatenation of *"M"* and _monthCode_.
       </emu-alg>
     </emu-clause>