Add JsonNumericMode parameter to jsonDecode for opt-in all-double parsing

[fabricio-costa]

Summary

• Adds JsonNumericMode enum to dart:convert with two values: preserveType (default) and allDouble
• Adds optional numericMode parameter to jsonDecode(), JsonCodec.decode(), and JsonDecoder
• When allDouble is used, all JSON numbers are parsed as double, aligning with ECMA-404's single "number" type

This is 100% backwards compatible — the default behavior is unchanged.

Motivation

jsonDecode('{"price": 5}') returns int for price, but jsonDecode('{"price": 5.0}') returns double. Since JSON makes no distinction between the two, this causes TypeError in Flutter apps when a backend returns a number without a decimal point for a field expected as double.

The current workaround ((json['x'] as num).toDouble()) must be applied manually to every double field in every model class. This proposal provides an opt-in solution at the parser level.

Related issue: #62776
See also: #46883, #42622, #55499, #56913

Implementation

The conversion happens at the listener level, not in parseNumber:

Platform	Change
VM	_JsonListener.handleNumber converts int → double when allDouble
JS	Wraps JSON.parse post-processing with int→double synthetic reviver
WASM	_JsonListener.handleIntegerNumber short-circuits to double

This keeps the performance-critical parsing code untouched. Zero overhead when using the default preserveType mode.

Usage

// Opt-in: all numbers become double
final data = jsonDecode(
  '{"price": 29, "quantity": 3}',
  numericMode: JsonNumericMode.allDouble,
);
print(data['price'] is double); // true

// Default: unchanged behavior
final data2 = jsonDecode('{"price": 29}');
print(data2['price'] is int); // true

Files changed

• sdk/lib/convert/json.dart — public API (enum + parameters)
• sdk/lib/_internal/vm/lib/convert_patch.dart — VM backend
• sdk/lib/_internal/js_runtime/lib/convert_patch.dart — JS backend
• sdk/lib/_internal/wasm/lib/convert_patch.dart — WASM backend
• tests/lib/convert/json_numeric_mode_test.dart — test coverage

Test plan

• Existing tests/lib/convert/json_* tests pass (backwards compat)
• New json_numeric_mode_test.dart covers:
  • Default preserveType behavior unchanged
  • allDouble converts integers to double (basic, zero, negative, large)
  • Scientific notation handled correctly
  • Non-numeric values (null, bool, string) unaffected
  • Nested structures (objects, arrays, deeply nested)
  • Reviver integration (receives converted doubles)
  • JsonCodec.decode() and JsonDecoder constructor paths

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[copybara-service]

Thank you for your contribution! This project uses Gerrit for code reviews. Your pull request has automatically been converted into a code review at:

https://dart-review.googlesource.com/c/sdk/+/483940

Please wait for a developer to review your code review at the above link; you can speed up the review if you sign into Gerrit and manually add a reviewer that has recently worked on the relevant code. See CONTRIBUTING.md to learn how to upload changes to Gerrit directly.

Additional commits pushed to this PR will update both the PR and the corresponding Gerrit CL. After the review is complete on the CL, your reviewer will merge the CL (automatically closing this PR).