Fix missing font constants on iOS7

Reviewed By: dinhviethoa

Differential Revision: D3682528

fbshipit-source-id: 7e3a0b7c2c043a869f2f5dfe9aaf404897076d0b

React/Views/RCTFont.mm

@@ -11,6 +11,25 @@
 
 #import <mutex>
 
+#if !defined(__IPHONE_8_2) || __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_8_2
+
+// These constants are defined in iPhone SDK 8.2, but the app cannot run on
+// iOS < 8.2 unless we redefine them here. If you target iOS 8.2 or above
+// as a base target, the standard constants will be used instead.
+// These constants can only be removed when React Native drops iOS8 support.
+
+#define UIFontWeightUltraLight -0.8
+#define UIFontWeightThin -0.6
+#define UIFontWeightLight -0.4
+#define UIFontWeightRegular 0
+#define UIFontWeightMedium 0.23
+#define UIFontWeightSemibold 0.3
+#define UIFontWeightBold 0.4
+#define UIFontWeightHeavy 0.56
+#define UIFontWeightBlack 0.62
+
+#endif
+
 typedef CGFloat RCTFontWeight;
 static RCTFontWeight weightOfFont(UIFont *font)
 {