Clarify current ResolveAbstractType algorithm

add assertions from reference implementation

Author: yaacovCR

spec/Section 6 -- Execution.md

@@ -734,9 +734,22 @@ the {objectValue}.
 
 ResolveAbstractType(abstractType, objectValue):
 
-- Return the result of calling the internal method provided by the type system
-  for determining the Object type of {abstractType} given the value
-  {objectValue}.
+- Let {resolvedType} be the result of calling the internal method provided by
+  the type system for determining the Object type of {abstractType} given the
+  value {objectValue}.
+- Assert {resolvedType} is an Object type.
+- If {IsSubType(abstractType, resolvedType)}:
+  - Return {resolvedType}.
+- Otherwise, raise a field error.
+
+IsSubType(superType, maybeSubType):
+
+- If {abstractType} is an Interface type.
+  - If {resolvedType} implements {abstractType}, return {true}.
+  - Otherwise, return {false}.
+- If {abstractType} is a Union type.
+  - If {resolvedType} is a member type of {abstractType}, return {true}.
+  - Otherwise, return {false}.
 
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