support per-component polyfill fallbacks

Author: guybedford

document/js-api/index.bs

@@ -459,7 +459,7 @@ To <dfn>instantiate imported strings</dfn> with module |module| and |importedStr
         1. [=map/set|Set=] |builtinOrStringImports|[|importedStringModule|] to |exportsObject|
     1. Let |imports| be « ».
     1. [=list/iterate|For each=] (|moduleName|, |componentName|, |externtype|) of [=module_imports=](|module|),
-        1. If |builtinOrStringImports| [=map/exist|contains=] |moduleName|,
+        1. If |builtinOrStringImports| [=map/exist|contains=] |moduleName| and |builtinOrStringImports|[|moduleName|] [=map/exist|contains=] |componentName|,
             1. Let |o| be |builtinOrStringImports|[|moduleName|]
         1. Else,
             1. Let |o| be [=?=] [$Get$](|importObject|, |moduleName|).