cleanup: move fieldType constants and functions to a separate file

lib/src/protobuf/wire_format.dart

Index: lib/src/protobuf/wire_format.dart
diff --git a/lib/src/protobuf/wire_format.dart b/lib/src/protobuf/wire_format.dart
index 6b6b1bdafcf9a9cd95469837fe7bfe3ce0ebcd60..daa2a706bc28a2afa0b48c6c36f0251f9ba6de58 100644
--- a/lib/src/protobuf/wire_format.dart
+++ b/lib/src/protobuf/wire_format.dart
@@ -19,3 +19,37 @@ int getTagFieldNumber(int tag) => (tag & 0x7fffffff) >> TAG_TYPE_BITS;
 int getTagWireType(int tag) => tag & TAG_TYPE_MASK;
 
 int makeTag(int fieldNumber, int tag) => (fieldNumber << TAG_TYPE_BITS) | tag;
+
+/// Returns true if the wireType can be merged into the given fieldType.
+bool _wireTypeMatches(int fieldType, int wireType) {
+  switch (FieldType._baseType(fieldType)) {
+    case FieldType._BOOL_BIT:
+    case FieldType._ENUM_BIT:
+    case FieldType._INT32_BIT:
+    case FieldType._INT64_BIT:
+    case FieldType._SINT32_BIT:
+    case FieldType._SINT64_BIT:
+    case FieldType._UINT32_BIT:
+    case FieldType._UINT64_BIT:
+      return wireType == WIRETYPE_VARINT ||
+          wireType == WIRETYPE_LENGTH_DELIMITED;
+    case FieldType._FLOAT_BIT:
+    case FieldType._FIXED32_BIT:
+    case FieldType._SFIXED32_BIT:
+      return wireType == WIRETYPE_FIXED32 ||
+          wireType == WIRETYPE_LENGTH_DELIMITED;
+    case FieldType._DOUBLE_BIT:
+    case FieldType._FIXED64_BIT:
+    case FieldType._SFIXED64_BIT:
+      return wireType == WIRETYPE_FIXED64 ||
+          wireType == WIRETYPE_LENGTH_DELIMITED;
+    case FieldType._BYTES_BIT:
+    case FieldType._STRING_BIT:
+    case FieldType._MESSAGE_BIT:
+      return wireType == WIRETYPE_LENGTH_DELIMITED;
+    case FieldType._GROUP_BIT:
+      return wireType == WIRETYPE_START_GROUP;
+    default:
+      return false;
+  }
+}