Sum and average (#11108)

This is implementation but not public release of sum and average. The sum/avg API is not made public. There are no API changes in this PR.

Author: MarkDuckworth

Firestore/Example/Firestore.xcodeproj/project.pbxproj

@@ -73,4 +73,12 @@ FIRQuerySnapshot *FSTTestQuerySnapshot(
 }  // extern "C"
 #endif
 
+@interface FSTNSExceptionUtil : NSObject
+
++ (BOOL)testForException:(void (^)(void))methodToTry
+          reasonContains:(nonnull NSString *)message
+    NS_SWIFT_NAME(testForException(_:reasonContains:));
+
+@end
+
 NS_ASSUME_NONNULL_END

Firestore/Example/Tests/API/FSTAPIHelpers.h

@@ -163,4 +163,18 @@
                                             metadata:std::move(metadata)];
 }
 
+@implementation FSTNSExceptionUtil
+
++ (BOOL)testForException:(nonnull void (^)())methodToTry
+          reasonContains:(nonnull NSString *)substring {
+  @try {
+    methodToTry();
+    return NO;
+  } @catch (NSException *exception) {
+    return [[exception reason] containsString:substring];
+  }
+}
+
+@end
+
 NS_ASSUME_NONNULL_END