Revert part of Nat encode improvement (#87)

Author: AlexV525

packages/agent_dart_base/lib/agent/utils/leb128.dart

@@ -22,12 +22,7 @@ List<T> safeRead<T>(BufferPipe<T> pipe, int ref) {
 /// nearest integer.
 /// @param value The number to encode.
 Uint8List lebEncode(dynamic value) {
-  BigInt bn = switch (value) {
-    BigInt() => value,
-    num() => BigInt.from(value),
-    String() => BigInt.parse(value),
-    _ => throw ArgumentError('Invalid big number: $value', 'lebEncode'),
-  };
+  BigInt bn = value is BigInt ? value : BigInt.from(value);
   if (bn < BigInt.zero) {
     throw StateError('Cannot leb-encode negative values.');
   }
@@ -63,13 +58,9 @@ BigInt lebDecode<T>(BufferPipe<T> pipe) {
 /// Encode a number (or bigint) into a Buffer, with support for negative numbers.
 /// The number will be floored to the nearest integer.
 /// @param value The number to encode.
-Uint8List slebEncode(dynamic value) {
-  BigInt bn = switch (value) {
-    BigInt() => value,
-    num() => BigInt.from(value),
-    String() => BigInt.parse(value),
-    _ => throw ArgumentError('Invalid big number: $value', 'slebEncode'),
-  };
+Uint8List slebEncode(Comparable value) {
+  BigInt bn = value is BigInt ? value : BigInt.from(value as num);
+
   final isNeg = bn < BigInt.zero;
   if (isNeg) {
     bn = -bn - BigInt.one;

packages/agent_dart_base/lib/candid/idl.dart

@@ -533,9 +533,7 @@ class NatClass extends PrimitiveType {
 
   @override
   bool covariant(x) {
-    return (x is BigInt && x >= BigInt.zero) ||
-        (x is int && x >= 0) ||
-        (x is String && BigInt.parse(x) >= BigInt.zero);
+    return (x is BigInt && x >= BigInt.zero) || (x is int && x >= 0);
   }
 
   @override

packages/agent_dart_base/test/agent/utils/leb128.dart

@@ -24,7 +24,6 @@ void leb128Test() {
       lebEncode(BigInt.from(60000000000000000)).toHex(),
       '808098f4e9b5ca6a',
     );
-    expect(lebEncode('1').toHex(), '01');
 
     expect(lebDecode(BufferPipe<int>(Uint8List.fromList([0]))), BigInt.zero);
     expect(lebDecode(BufferPipe<int>(Uint8List.fromList([1]))), BigInt.one);
@@ -60,7 +59,6 @@ void leb128Test() {
       slebEncode(BigInt.parse('60000000000000000')).toHex(),
       '808098f4e9b5caea00',
     );
-    expect(slebEncode('1').toHex(), '01');
 
     expect(
       slebDecode(BufferPipe<int>(Uint8List.fromList([0x7f]))),